Replace polling loop with NATS pub/sub for game updates

Use via's embedded NATS server to notify players of state changes
instead of a 100ms polling ticker. Each player subscribes to
"game.<id>" on page load; via auto-cleans subscriptions on disconnect,
eliminating the need for manual player tracking and RegisterSync.

main.go

@@ -15,6 +15,7 @@ import (
 	"github.com/ryanhamamura/c4/ui"
 	"github.com/ryanhamamura/via"
 	"github.com/ryanhamamura/via/h"
+	"github.com/ryanhamamura/via/vianats"
 )
 
 var store = game.NewGameStore()
@@ -43,12 +44,20 @@ func main() {
 		log.Fatal(err)
 	}
 
+	ctx := context.Background()
+	ns, err := vianats.New(ctx, "./data/nats")
+	if err != nil {
+		log.Fatal(err)
+	}
+	store.SetPubSub(ns)
+
 	v := via.New()
 	v.Config(via.Options{
 		LogLevel:       via.LogLevelDebug,
 		DocumentTitle:  "Connect 4",
 		ServerAddress:  ":7331",
 		SessionManager: sessionManager,
+		PubSub:         ns,
 		Plugins:        []via.Plugin{DaisyUIPlugin},
 	})
 
@@ -307,7 +316,6 @@ func main() {
 				}
 				gi.Join(&game.PlayerSession{
 					Player: player,
-					Sync:   c,
 				})
 			}
 			c.Sync()
@@ -336,6 +344,11 @@ func main() {
 			}
 		})
 
+		// Subscribe to game updates so the opponent's moves trigger a re-render
+		if gameExists {
+			c.Subscribe("game."+gameID, func(data []byte) { c.Sync() })
+		}
+
 		// If nickname exists in session and game exists, join immediately
 		if gameExists && sessionNickname != "" && gi.GetPlayerColor(playerID) == 0 {
 			player := &game.Player{
@@ -347,11 +360,7 @@ func main() {
 			}
 			gi.Join(&game.PlayerSession{
 				Player: player,
-				Sync:   c,
 			})
-		} else if gameExists && gi.GetPlayerColor(playerID) != 0 {
-			// Re-register sync context for existing players on reconnect
-			gi.RegisterSync(playerID, c)
 		}
 
 		c.View(func() h.H {